A common fuel assembly design providing the fissionable fuel core for power generating, water cooled nuclear reactors comprises a multiplicity of so-called "bundles", or composite units of interconnected containers of fuel, which are assembled in predetermined patterns. Each bundle typically comprises a group of sealed tubes or elongated container partially filled with fissionable fuel materials, which are aligned parallel and spaced apart from each other in a given design. Bottom and top mountings, and intermediate spacer devices fix the relative positions of the fuel container array. Each such group of fuel containers and their securing devices is surrounded with an open-ended metal circumferential enclosure, referred to in the industry as a "channel". Channels for fuel containers most commonly are square in cross-section for service in power generating, water cooled and moderated nuclear reactors, although they can be of any given cross-section, for example hexagonal for some liquid metal cooled reactors.
A detailed illustration of a typical fuel channel of generally square cross-section, comprising an elongated circumferential enclosure surrounding a group of fuel tubes or rods and forming a bundle, is illustrated in U.S. Pat. Nos. 3,350,275, and 3,689,358 issued Oct. 31, 1967 and Sept. 5, 1972, respectively. The disclosed contents of these patents are accordingly incorporated herein by reference.
Such fuel bundle channel units are commonly fabricated by bending both longitudinal edges of an elongated strip of metal into a U-shape piece. Then two U-shaped pieces are matched together with their longitudinal edges adjoining and welded along each of the adjoined longitudinal edge to unite the two half pieces into a unitary open-ended, circumferential enclosure. The typical channel unit is of generally square cross-section with four equal sides, for example about 6 inches across, produced by uniting two elongated half sections, each formed from a single metal strip.
A substantially inevitable aspect of such a fabrication process is that the resulting product, namely a long, open-ended metal square or rectangular duct, is typically somewhat distorted and not precisely symmetrical, and lacks accurate and uniform dimensions with deviating flatness of surfaces.
In the channel units designated functions as a component of fuel bundles for service in the fuel core assembly of nuclear reactors, accuracy of dimensions, good symmetry, and a distortion free configuration with flat plane surfaces are all highly significant requirements. Exposure of the channels in a reactor to high and uneven levels of radiation and heat with the resultant potential for deformation due to relaxation of residual stresses is amplified by the neutron irradiation encountered within a nuclear reactor. These changes can distort tolerances which are required for the interaction of fuel core components, such as control rods, in reactor operation. Moreover, the necessary securing of a channel unit at several locations of its body to other components, coupled with these inevitable dimensional changes, compounds the distortion and may extend the effect to other components.